You need a protractor and a dti (dial gauge) to do this accurately, essential tools for the engine builder.

There is a 'dwell' period around top centre (TDC) where the piston doesn't move, the bigger the engine the bigger the dwell phase; it can be 12 crank degrees or more, use the protrator to find the mid position.Don't even think about building a race engine without taking this kind of care over the build.

The method of finding piston true tdc is exactly the same as finding full lift on the cam, shown at:

True TDC on a 2.3 HSR block. With the head off this is easy - with the head on - put a probe thru the spark plug hole but on inclined valve engines like this make sure the valves don't foul your probe.